Chemical mixture separation is important to many fields. While a variety of approaches exist for chemical separations, chromatography and electrophoresis are two of the most commonly used analytical methods.
Chromatography is a set of laboratory techniques used to separate constituents from a chemical mixture. Chromatography is used in everything from water and food safety to biotechnology and drug discovery. It is also a common technique used in standard laboratory procedures and cutting-edge scientific research, where liquid chromatography, size-exclusion chromatography, affinity chromatography and high-pressure liquid chromatography are traditionally used.
In liquid chromatography, the unknown sample is dissolved in a liquid mobile phase, which is then over a stationary phase. The analyte of interest remains with, or is slowed by, the stationary phase, separating it from the overall mobile phase mixture.
Liquid chromatography can be further divided by the stationary phase, comprising three methods: ion-exchange, size-exclusion, and affinity chromatography. In ion-exchange chromatography, the stationary phase contains charged functional groups, which interact with the charge on the analyte. This charge will affect the migration time through the chromatography system, separating the analyte from the overall sample.
Size-exclusion chromatography, or gel permeation chromatography, separates constituents based on size, passing the mobile phase through a porous medium that only passes particles below a certain size.
Affinity chromatography is based on selective covalent bonding with the mobile phase. For example, proteins or polymers tagged with a specific linker can be isolated with the appropriate linker analogue.
General improvements in liquid chromatography have increased the efficiency and resolution, leading to the more common description of high-pressure (or performance) liquid chromatography (HPLC). Such separations typically require macroscopic volumes of material—the mobile phase may be a few milliliters or more. Additionally, these separations can take hours to process. And, even with these drawbacks, the results are still rather crude single-monomer polymer separation across a broad stripe of analytes is a significant challenge.
An alternative to HPLC is electrophoresis, where charged molecules are separated in an electric field. FIG. 1 shows a prior art schematic illustration of electrokinetic flows 100. The electroosmotic 102 and electrophoretic 104 flows of particles 106 scale linearly with the electric field E by a mobility factor μ inside a channel 108, where the electric field is created by a voltage supply 110 across the length of the channel 108. The arrows are shown in opposite directions for illustration, where they may point in either direction depending on charges and material properties. This separation of the components of a mixture of charged molecules is an important scientific and technological process, including analytical methods such as DNA sequencing and preparative methods such as the purification of proteins. Successful separation of a mixture of polyelectrolytes by an applied electric field according to charge or mass depends on symmetry-breaking mechanisms between the driving force, related to the electric field, and the friction offered by the medium, such as a buffer solution with or without a matrix such as a gel. Accordingly, if the driving force and the friction force scale the same way with charge or length, the ratio of these quantities is then independent of charge or length, and separation is not achieved. There are many ways to achieve symmetry-breaking for polyelectrolytes such as DNA, ranging from the use of gel matrices for sequencing relatively short DNA fragments in a constant electric field, to pulsed-field gel electrophoresis for separating large DNA fragments, or to creating asymmetric molecules for separation in free solution.
There are multiple approaches to electrophoresis. Capillary electrophoresis separates components within a glass capillary. Here, the properties of microfluidic flow within such a capillary improve efficiency and reduce separation times. Known instruments can reduce the separation time to 30 minutes, while allowing the use of a few microliters of material. For fields where materials are expensive and rare, such as drug discovery, capillaries offer tremendous benefits.
Furthermore, in the last few years, microfabricated capillary electrophoresis devices have entered the market. These devices offer parallel processing with a few to dozens of simultaneous separations. Microfabricated devices also work with smaller sample sizes. These advantages are both increasingly important in biotechnology, as many samples are of limited material quantities. Commercial microfabricated capillary electrophoresis systems are being used for DNA sequencing, RNA analysis, protein separations, and even cell content studies.
The ability to separate a chemical mixture into constituents is absolutely necessary for all of analytical chemistry. Improvements in speed, quality, efficiency, or resolution of separation techniques are necessary enhance the behind-the-scenes laboratory work that ensures the quality of everyday products. These products can be categorized in the fields of pharmaceuticals, laboratory, environmental, food/beverage, and academic. Each of these segments has a broad impact across all of society, such as quality-control analysis, where imported food products must be analyzed for hazardous materials, or water and soil must be measured for pollutants before entering public consumption.
Many liquid chromatography methods are used in environmental analysis, such as water and soil quality analyses, where measuring organic compounds or mineral-content levels in water is handled by experienced lab technicians operating chromatography tools. These processes are expensive and time-consuming. Nevertheless, the application of chromatography in these fields is tremendous and pervasive. Thus, the broader impact of new techniques to speed and improve chemical separations is wide-ranging and important. While recent electrokinetic separations have improved separation speed and resolution for charged molecules over traditional chromatographic techniques, such improvements are still lacking for many particles including charged and uncharged molecules.
Capillary electrophoresis provides improvements in speed and resolution over LC. Furthermore, capillary electrophoresis works effectively in parallel systems and with microscopic volumes. When molecules are uncharged, electrophoretic methods have been ineffective, the options for separating such molecules are limited to older LC techniques, such as size-exclusion or affinity chromatography.
Uncharged polymers are important in many everyday products. For example, poly(ethylene glycol) is used in a multitude of medical applications: in laxatives, in skin creams and eye drops, and for delayed protein drug delivery. The polymer poly(vinyl alcohol) is used extensively in products ranging from children's putty to adhesives. Furthermore, under electrophoretic conditions, free-draining coils, such as DNA, are effectively uncharged as their drag-to-charge ratio is uniform, where DNA will not separate in an electric field without a symmetry-breaking mechanism. Additionally, many proteins and peptides are effectively uncharged; electrophoretic separation of these important biomaterials is not possible with additional processing steps. As uncharged polymers are necessary components of everyday materials, improved separations of these materials will improve the safety and quality of these products.
Accordingly, there is a need to develop hand-held low-cost microfluidic separation device separating charged and uncharged particles, where the method has broad applications in environmental, biotechnological, and chemical processing. A further need exists for such a device that provides detection resolution at the part-per-billion (ppb) level.